Nevertheles, Van Dobben and Ter Braak (1998) found that in field monitoring it is very difficult to separate the effects of many intercorrelated variables and this is especially true for pollution studies, as pollutant concentration trend to be correlated to the general level of human activity and are therefore correlated among each other. Which, correspond the work of Munzi et al. (2007) that found besides air pollution, the most important variable affecting the epiphytic lichen flora of Rome, currently updated to 102 taxa, is the influence of the Tyrrhenian Sea.